July 3, 2014

Distant Cannibal Provides Important Clues On How Galaxies Grow

Astronomers are hoping to gain a new understanding of how galaxies grow from watching a distant “twin” of the Milky Way as it is swallowing another galaxy.

Dr. Caroline Foster of the Australian Astronomical Observatory (AAO) led the team that has been studying NCG 4651, called the Umbrella galaxy for the “parasol” of stars that are all that remain of the galaxy being consumed. Umbrella can be found approximately 62 million light-years from Earth in the northern constellation of Coma Berenices. Their findings are to be published in the Monthly Notices of the Royal Astronomical Society.

Astronomers using the Asian-Australian Telescope (AAT) two decades ago identified a “new” dwarf galaxy, named Sagittarius, being consumed by our own Milky Way Galaxy.

This was the first evidence that astronomers found of the Milky Way acquiring stars by snacking on other, smaller, galaxies. This discovery has led to observations of stellar streams in other galaxies as well.

David Martinez-Delgado of the University of Heidelberg, in 2010, led a study that used small robotic telescopes to image eight isolated spiral galaxies. His team found signs of mergers (shells, clouds, and arcs of tidal debris) in six of the eight.

Martinez-Delgado hypothesized that the distinctive arc of the Umbrella galaxy was a consequence of a single merger, rather than several events over a long time period. The current study, which used data from the Subaru and Keck telescopes in Hawaii, supports this theory.

“Through new techniques we have been able to measure the movements of the stars in the very distant, very faint, stellar stream in the Umbrella,” Dr. Foster said. “This allows us to reconstruct the history of the system, which we couldn’t before.”

Dr. Aaron Romanowsky of San Jose State University and the University of California Observatories commented that being able to study streams this far out means that many more galaxies can be put under the microscope.

“In turn that means we can get a handle on how often these ‘minor mergers’ — an important way that galaxies grow — actually occur,” he said.

The team used three sets of tracers—clusters of old stars (globular clusters); old, brightly glowing stars (planetary nebulae); and patches of glowing hydrogen gas (HII regions)—to determine the movement of the stars in the stream.